In order to optimize efficiency of a power amplifier one needs to keep a supply voltage of the power amplifier as low as possible; in this document the efficiency means: output signal power/(input signal power+energizing power of the power amplifier). On the other hand the supply voltage has to be high enough that no clipping occurs in an output signal of the power amplifier. In an envelope tracking power amplifier an envelope of an input signal of the amplifier is detected and the supply voltage is a function of the detected value of the envelope. The power amplifier operates close to a compression point on all envelope values and thus provides a good efficiency. Keeping the supply voltage at an optimal value is a challenging task especially when the output signal of the power amplifier has a high peak to average ratio. A value of the supply voltage has to be able to follow variations in the value of the envelope so closely that no clipping occurs and, on the other hand, efficiency of the power amplifier is sufficiently good. In order to fulfil this requirement a controllable voltage source that produces the supply voltage has to have a sufficiently high bandwidth. High bandwidth of the controllable voltage source means, however, high power losses in the controllable voltage source. For example, in a case of a switched mode power supply (SMPS) as the controllable voltage source a switching frequency has to be increased when the bandwidth is increased. Increasing the switching frequency increases switching losses. A simple solution for taking quick variations in the envelope into account is to keep the value of the supply voltage so high that there is a sufficient safety margin in different situations. This kind of approach leads, however, to a situation in which the supply voltage is unnecessarily high over a significant portion of time. Too high a supply voltage means unnecessary power losses especially in output stage transistors of a power amplifier.